This invention relates to overclad fiber optic couplers that are capable of withstanding relatively wide temperature excursions and to a method of making them.
Fiber optic couplers referred to as "fused fiber couplers" have been formed by positioning a plurality of fibers in a side-by-side relationship along a suitable length thereof and fusing the claddings together to secure the fibers and reduce the spacings between the cores. Various coupler properties can be improved by encapsulating the coupling region of the fibers in a matrix glass to form an "overclad coupler". Portions of the fibers to be fused are inserted into a glass tube having a refractive index lower than that of the fiber claddings. The tube has a longitudinal bore, each end of which is connected to the tube end surfaces by a funnel that facilitates the insertion of the fibers. The tube midregion is collapsed onto the fibers; the central portion of the midregion is then drawn down to that diameter and coupling length which is necessary to obtain the desired coupling. After the coupler has cooled, a drop of glue is applied to each funnel to increase the pull strength of the fibers.
The collapsing of the tube midregion causes the optical fibers just outside the collapsed region to be weakened. This weakened region occurs about 5 mm from the fully collapsed region in the uncollapsed portion of the bore. Under severe thermal cycling (for example, cycling a coupler between -60.degree. C. and 125.degree. C.) the weakened region of the fibers has been known to break, primarily because of a thermal coefficient of expansion mismatch between the glue and the glass coupler components. Because of the angle of the funnel walls with respect to the bore axis, this thermal expansion mismatch causes the glue in the funnel to expand longitudinally outwardly and pull the fibers embedded therein away from the collapsed midregion, thereby stressing the fibers.
Heretofore, the filling of the cavity space with glue has been considered to be desirable, but the air trapped inside the tube bore has kept the glue from penetrating into the bore. A technique for increasing the depth of penetration of the glue into the uncollapsed portion of the tube bore is disclosed in U.S. Pat. No. 5,009,692. A drop of glue is applied to an end of the coupler tube. Heat is applied locally to a lateral end region of the tube in the vicinity of the uncollapsed bore. As the air within the bore becomes heated, it expands and is driven from the bore, causing bubbles to pass through the glue. After the heat source is removed, the air remaining in the bore cools and draws the glue a short distance into the bore. However, the technique of heating the tube and then allowing the glue to be drawn into the bore does not cause the glue to consistantly flow far enough into the bore to adequately cover or relieve stress from the weakened region of the fibers.
It is noted that fully filling the funnel and the uncollapsed portion of the bore with glue would be effective in reducing harmful stresses that otherwise would be applied to the fibers only if the glue is azimuthally homogeneous with respect to the fibers. The presence of a large void in the glue can cause the fibers to be stressed.